Centrifugal pump with magnetic coupling

ABSTRACT

A motor driven centrifugal pump with magnetic transmission comprising a permanent magnetic pole ring and a soft iron magnetic pole ring whereby means are provided for reducing the magnetic flux which determines the torque and which flows through the two pole rings.

United States Patent Laing Mar. 14, 1972 [54] CENTRIFUGAL PUMP WITH [56]References Cited MAGNETIC COUPLING UNITED STATES PATENTS [72] Inventor:Nikolaus Laing 7141 Aldingen near Stuttg3", Hofencr 3537 Germany3,354,833 11/1967 La ng ..310/104 X 3,490,379 l/ 1970 Lamg [22] Filed:Nov. 30, 1970 [21] Appl. No.: 93,826 Primary Examiner-Robert M. Walker[52] US. Cl .417/4'20, 310/104 [571 ABSTRACT [51] Int. Cl. ..F04b 17/00,F04b 35/04, H02k 5/10, A motor driven centrifugal pump with magnetictransmission 9 49/00 15/00 comprising a permanent magnetic pole ring anda soft iron [58] Field ofSearch .417/420, 3153;310/103, 104,

310/105 magnetic pole ring whereby means are provided for reducing themagnetic flux which determines the torque and which flows through thetwo pole rings.

28 Claims, 10 Drawing Figures FIG. 7

Patented March 14, 1972 4 Sheets-Sheet 1 IN VEN TOR Patented March 14,1972 3,649,137

I 4 Sheets-Sheet 2 I m I m 1 INVENTOR Patented March 14, 1972 3,649,137

4 Sheets-Sheet :s

INVEN TOR Patented March 14, 1972 4 Sheets-Sheet 4 IN VEN TORCENTRIFUGAL PUMP WITII MAGNETIC COUPLING THE PRIOR ART Pumps with amagnetic transmission coupling have the advantage that the pump housingsare hermetically sealed by means of a magnetically permeable separatingwall. They have the further advantage that they prevent a predeterminedtorque being exceeded, e.g., when conveying liquids of excessiveviscosity or in the case of ingress of clogging solid bodies, by the useof permanent or hysteresis magnets for one of the pole rings of thecoupling and a permanent magnet for the other pole ring of the coupling,in that the coupling drops out of synchronism and thereafter no longertransmits any torque. With a view to adapting the conveyancecharacteristics to the resistance characteristics, the diameter of therunner is, in the case of all larger pumps, reduced by turning on alathe until the throttle characteristics which is a function of therunner diameter cuts the resistance characteristic of the power systemat the desired operating point. This method of adaptation isirreversible, and if the resistance in the power system is subsequentlyincreased, a new pump runner has to be installed.

With a view to avoiding this disadvantage, many pumps are provided witha controllable bypass. The lower the desired operating pressure, thegreater is the proportion of the throughput which is returned from thepressure side to the suction side through the bypass. Against thisadvantage of adjustability there is the disadvantage of low hydraulicefficiency. The smaller the product the pressure X quantity conveyed bythe pump, i.e., the lower the hydraulic power provided, the greater isthe total power demand and thus, for example, the electric powerconsumption of the driving motor. Particularly in the case of pumpswhich are operated contunuously and over long periods, this adjustablecontrol results in a considerable increase in energy consumption ascompared with the conventional method first described.

DESCRIPTION OF THE INVENTION The invention avoids these disadvantages.The runners of the pumps in accordance with the invention are driventhrough a magnetic coupling which consists only of a permanent magnetpole ring and a soft magnetic pole ring which preferably has a squirrelcage winding or a layer of material of high electric conductivity.Furthermore the invention uses means whereby the magnetic flux whichdetermines the torque and which flows through the two pole rings can bereduced. This can be done in the following manner:

a. by increasing the air gap between the pole rings b. by producing orincreasing an eddy current in the separating wall 0. by partiallyshort-circuiting adjacent poles by means of soft iron or permanentmagnets d. by increasing the magnetic reluctance in the return pathregion between adjacent poles.

The like permanent magnet pole rings of a magnetic coupling incombination with a second permanent magnet pole ring driving the runneror also a soft magnetic pole ring with salient geometric poles or also apole ring made of hysteresis material enable the stalling torque to beadjusted, so that one and the same coupling may be used for motors ofdifferent maximum torques.

Alternative embodiments provide adjustability during operation andduring stadstill of the pump respectively. Adjustability of the slip oralso of the stalling torque while in operation entails much moreequipment, but is also required only on very rare occasions.

The invention will be described with reference to the drawings.

FIG. I shows the circulating pump with a motor shaft which is axiallyslidable during operation;

FIG. 2 shows a pole ring axially slidable on the shaft during standstillonly by means of a cam;

FIG. 3 shows a housing construction with inclined slotted guides foraxial displacement of the entire drive assembly relative to the pumphousing;

FIG. 4 shows a housing construction with axial displacement of thedriving assembly by means of a set screw;

FIGS. 5a and 5b show a magnetic coupling in accordance with theinvention with a magnetic shunt which is adjustable in azimuth;

FIG. 6 shows another form of magnetic coupling with a magnetic shuntadjustable in azimuth;

FIG. 7 shows a permanent magnetic pole ring having a disc with soft ironsegments arranged at its axial end;

FIG. 8 shows a permanent magnetic pole ring with circumferentiallyslidable soft iron pieces;

FIG. 9 shows a permanent magnetic pole ring with series connectedaxially magnetized spherical segments.

FIG. I shows a centrifugal pump in accordance with the invention, insection. The rotor 2, which forms a unit with a soft iron pole ring 3,rotates in the pump housing I. The soft iron core of the pole ring 3 isenclosed by a copper shell 4, which serves simultaneously the purpose ofproducing eddy currents and of providing protection for the soft ironpole ring against corrosion. The motor housing 6 is secured to the pumphousing 1 via a moulded rubber ring 5 which provides a positiveconnection and a measure of sound proofing.

The motor housing 6 contains a motor whose armature 9 is supported inthe end plates 7 and 8. The end plates are provided with ribs whichpositively locate the motor centrally in the motor housing 6.

The concave pole ring 12 is built up of permanent magnet segments and isdriven by the armature 9 via the shaft 13. The ball bearing 14 isslidably arranged in the bushing 15 of the end plate 8 and is biasedtowards the motor by the coiled spring 17. By means of a distance ring17 the inner race of the ball bearing 14 is positively held on the shaft13. The ball bearing 18 is also supported in the bushing 19 of the endplate 7 so as to be slidably displaceable with the outer ring. The outerring is positively secured to the screw 21 by a cup shaped intermediatemember 20 by means of securing means 22. A screw 21 has a collar 23which can be fixed axially by inserting annular shims 24. In theposition shown the air gap between the separating shell 25 and theconcave pole ring 12 is at its narrowest. If the screw 21 is turnedoutwards, the spring 16 causes displacement of the armature-pole ringunit 9, 12 whereby the air gap between the inner shell 25 and the polering 12 is increased. The greater the air gap is made, the greaterbecomes the slip. It has been found that the largest air gap at whichthe runner 2 still rotates in a stable manner, the speed of the polering-runner unit 2,3 drops to approximately one-third of the nominalspeed, whereby the hydraulic power of the pump is reduced to onetwenty-seventh whilst the power required of the motor is reduced toone-ninth. In this way a significant saving in electricity cost isachieved. The motor is cooled by means of a fan ring 26, which isattached to the soft iron return path cap 27 of the pole ring 12 andforms a gap 28 with the motor housing 6. Cooling air enters at 29 and isdischarged through the punched apertures 30.

The armature 9 is so associated with the stator 6 that it lies flush inthe stator when the shoulder 23 abuts the screw 21. As the coupling isreduced, the armature 9 is displaced out of its stator 6.

FIG. 2 shows a different solution in which the distance of the outerpole ring 12, from the separating shell 25 is variable via a hub 33which is slidably displaceable in an axial direction on, and securableto, the motor shaft 34. Adjustment in the axial direction isaccomplished by a cam 35 which slides in an elongated hole 36. The polering is secured by means of set screws 37. By contrast with theembodiment shown in FIG. I, this device can only be adjusted duringstandstill.

FIG. 3 shows a further embodiment of the invention. A flange 37 in whichthe entire motor housing 38 is axially displaceable is secured to thepump housing 1. Axial adjustment is accomplished by slight tilting ofthe motor housing 38 in the slotted guides 39, and fixing by tighteningup the screws 40. As in the case of the embodiment shown in FIG. 1, theaxial distance can, if desired, be varied whilst the pump is running.

FIG. 4 shows a further embodiment in which the motor 41 is suspended intwo rubber rings 42 and 43 which are secured to the motor housing 44 atits outer periphery in axially fixed relationship and exert an axialforce in the direction of the arrow 45. The adjusting screw 46, whichtransmits its force via the elastic rubber element 47 to the motor 41 soas to prevent the transmission of sound from the body, acts against thisaxial force. As in the case of the embodiment shown in FIG. 1, the axialdistance can here also be varied whilst the pump is running.

FIG. a shows diagrammatically the principle of construction of acylindrical coupling in which the permanent magnets 50 of the inner polering are arranged between the inner pole ring 52 and the outer pole ring53 transversely to the direction of the radial lines of force 51. Theiron segments 54 which lie between the magnets 51 serve as pole shoes.Starting from their outer surface 55 the lines of force extend acrossthe air gap 56 to the outer pole ring 53 which is made up in the form ofa squirrel cage rotor of soft iron 59 and conductor rods 60 which areshort-circuited together at their axial ends. The

magnitude of the active magnetic field in the air gap 56 can beinfluenced via the rear side 57 of the iron segment 54, in that byvarying the position of the iron parts 58 which are interconnected by amagnetically ineffective ring (not shown) in azimuth, a magnetic shuntbetween adjacent iron segments 54 and 54 can be created.

FIG. 5b shows the same construction of the inner pole ring, whereas theouter pole ring 53 consists of an iron ring and radially magnetizedmagnets 12 secured therein.

FIG. 6 shows a construction in which the iron parts 61 which aredisplaceable in azimuth serve the purpose of adjustably improving thereturn path of the magnetic lines of force, the return path beingprovided via the thin walled iron cylinder 62 in an unsatisfactorymanner, so that the effect of the force on the rotor 63 in the positionshown is at a maximum, and in a position displaced by half a pitch at aminimum. In this construction a hysteresis material was chosen for therotor 63 which has only secondary produced magnetic poles 66, so thatabove a maximum torque the couplings slip without vibration, whereasbelow that maximum torque it transmits the torque without slip.

FIG. 7 shows an axial plan and an axial plan as seen from the oppositedirection as well as an axial section of a pole ring for a coupling witha spherical air gap. The magnets 70 have iron segments 71 disposedbetween them which are enlarged by soft iron spherical segments 72towards the air gap. On the rear side a ring 73 made of nonmagneticmaterial is provided in which soft iron pieces 74 are inserted whosespan corresponds to approximately four-fifths of the pitch 75. In thesector 76 these soft iron pieces magnetically shortcircuit adjacent ironsegments, whilst they each register with a soft iron segment 71 in thesector 77, so that the magnetic short circuit disappears.

FIG. 8 shows a like embodiment, in which however a ring 80 is associatedwith soft iron pieces 81 which again connect adjacent iron segments 71or the neutral central zones of adjacent magnets 70/70 with each other.

FIG. 9 shows diagrammatically and partly in section an outer pole ringconsisting of a soft iron body 90 in which a first plurality of radiallymagnetized magnets 95/95 is secured. In the so formed spherical cup afurther plurality of axially magnetized spherical segments 91/91 isprovided which is collected together via nonmagnetic segments 92 into apivotable spherical cup. By pivoting this spherical cup by half a pitchas shown at 94/94 the magnetic fields of the magnetic segments 90 and 91are partially neutralized.

Iclaim:

1. A motor driven centrifugal pump whose runner forms a unit enclosed ina pump housing, with a soft magnetic pole ring, the wall of the pumphousing adjacent the pole ring consisting of magnetically permeablematerial and a permanent magnet pole ring driven by the motor andarranged to transmit the motor torque to the first pole ring with littleoperational slip being arranged adjacent and outside this wall,characterized in that means are provided for reducing the flux passingthrough the magnetically permeable separating wall, thereby the slip isincreased.

2. A centrifugal pump according to claim 1, characterized in that themagnetic air gap is variable by axial displacement of one pole ring 12)relative to the other.

3. A centrifugal pump according to claim 2, characterized in that thepole ring (12) is arranged so as to be displaceable relative to theshaft (13) and axially securable.

4. A centrifugal pump according to claim 2, characterized in that thepole ring (12) forms a unit with the shaft (13) and the armature (9) andthat this unit is arranged so as to be axially displaceable.

5. A centrifugal pump according to claim 4, characterized in that theshaft is supported in ball bearings and that the bearing (14) facing thepump housing is spring biased by the spring (16) towards the pole ring(6), whereas the second ball bearing (18) is axially adjustable bypositively connected elements (18, 21).

6. A centrifugal pump according to claim 4, characterized in that themotor armature (9) is disposed symmetrically to the central rotationalplane of the stator (6'), when the air gap of the pole ring (12) is at aminimum, whereas the motor armature is displaced from the optimumcentral position as the air gap increases.

7. A centrifugal pump according to claim 2, characterized in that themotor housing (38) is arranged so as to be displaceable relative to thepump housing (1).

8. A centrifugal pump according to claim 1, characterized in that thefirst pole ring (3) which is integral with the pump runner (2) consistsof soft iron and carries a surface layer (4) of a material of highelectrical conductivity.

9. A centrifugal pump according to claim 8, characterized in that thelayer of the material of high electrical conductivity is copper and thatit also protects the side of the rotor (3) facing the separating wall(27).

10. A centrifugal pump according to claim 1, characterized in that thefirst pole ring (3) which is integral with the pump runner (2) consistsof a magnetic material whose coercive force is so small that at leastduring acceleration when starting up slip occurs between the first andthe second pole ring (12).

11. A centrifugal pump according to claim 1, characterized in that aring (73, 80) of magnetically nonconductive material is associated withbodies (58, 74, 81) of soft iron, whose circumferential span is smallerthan the pitch (75) between adjacent magnet poles and that the ring (73,80) can be pivoted through an angle corresponding to approximately halfa pole pitch (75 12. A centrifugal pump according to claim 10,characterized in that the magnet poles (54, 71) consist of soft iron andthat the permanent magnets (50, 70) are magnetized in thecircumferential direction.

13. A centrifugal pump according to claim 1, having a pole ring withpermanent magnets, one pole face of the magnets being arranged facingthe air gap and the other pole face facing a magnetic return path ring,characterized in that the magnetic return path ring (62) has a number ofregions (61) of high magnetic conductivity equal to the number of poles,the said regions being arranged for displacement in azimuth in such away that in one terminal position thereof they are disposed between twoadjacent poles and there form the magnetic return path between adjacentpoles, whereas in their other terminal position they register with thepoles and thus do not present a return path. 7

14. A centrifugal pump according to claim 1, characterized in that thepole ring carrying permanent magnets (91, 95) consists of two ringsarranged within each other or adjacent each other and having the samenumber of poles and in that the rings are arranged for relativedisplacement by approximately half a pole pitch, so that in one terminalposition the magnetic fluxes are additive, whereas in the other terminalposition they partially neutralize each other.

15. A motor driven centrifugal pump whose runner forms a unit enclosedin a pump housing, with a soft magnetic pole ring, the wall of the pumphousing adjacent the pole ring consisting of magnetically permeablematerial and a permanent magnet pole ring driven bythe motor andarranged to synchronously transmit the motor torque to the first polering, characterized in that means are provided for reducing the fluxpassing through the magnetically permeable separating wall, whereby thestalling torque is reduced.

16. A centrifugal pump according to claim 15, characterized in that themagnetic air gap is variable by axial displacement of one pole ring (12)relative to the other.

17. A centrifugal pum'p according to claim 16, characterized in that thepole ring (12) is arranged so as to be displaceable relative to theshaft (13) and axially securable.

18. A centrifugal pump according to claim 17, characterized in that thepole ring (12) forms a unit with the shaft (13) and the armature (9) andthat this unit is arranged so as to be axially displaceable.

l). A centrifugal pump according to claim 18, characterized in that theshaft is supported in ball bearings and that the bearing (14) facing thepump housing is spring biased by the spring (16) towards the pole ring(6'), whereas the second ball bearing (18) is axially adjustable bypositively connected elements (18, 21).

20. A centrifugal pump according to claim 18, characterized in that themotor armature (9) is disposed symmetrically to the central rotationalplane of the stator (6), when the air gap of the pole ring (12) is at aminimum, whereas the motor armature is displaced from the optimumcentral position as the air gap increases.

21. A centrifugal pump according to claim 16, characterized in that themotor housing (38) is arranged so as to be displaceable relative to thepump housing (1).

22. A centrifugal pump according to claim 15, characterized in that thefirst pole ring (3) which is integral with the pump runner (2) consistsof soft iron and carries a surface layer (4) of a material of highelectrical conductivity.

23. A centrifugal pump according to claim 22, characterized in that thelayer of the material of high electrical conductivity is copper and thatit also protects the side of the rotor (3) facing the separating wall(27).

24. A centrifugal pump according to claim 15, characterized in that thefirst pole ring (3) which is integral with the pump runner (2) consistsof a magnetic material whose coercive force is so small that at leastduring acceleration when starting up slip occurs between the first andthe second pole ring (12).

25. A centrifugal pump according to claim 15, chara terized in that aring (73, of magnetically nonconductive material is associated withbodies (58, 74, 81) of soft iron, whose circumferential span is smallerthan the pitch (75) between adjacent magnet poles and that the ring (73,80) can be pivoted through an angle corresponding to approximately halfa pole pitch (75).

26. A centrifugal pump according to claim 24, characterized in that themagnet poles (54, 71) consist of soft iron and that the permanentmagnets (50, 70) are magnetized in the circumferential direction.

27. A centrifugal pump according to claim 15, having a pole ring withpermanent magnets, one pole face of the magnets being arranged facingthe air gap and the other pole face facing a magnetic return path ring,characterized in that the magnetic return path ring (62) has a number ofregions (61) of high magnetic conductivity equal to the number of poles,the said regions being arranged for displacement in azimuth in such away that in one terminal position thereof they are disposed between twoadjacent poles and there form the magnetic return path between adjacentpoles, whereas in their other terminal position they register with thepoles and thus do not resentareturn ath.

2 A centrifuga pump according to claim 15, characterized in that thepole ring carrying permanent magnets (91, consists of two rings arrangedwithin each other or adjacent each other and having the same number ofpoles and in that the rings are arranged for relative displacement byapproximately half a pole pitch, so that in one terminal position themagnetic fluxes are additive, thereas in the other terminal positionthey partially neutralize each other.

1. A motor driven centrifugal pump whose runner forms a unit enclosed ina pump housing, with a soft magnetic pole ring, the wall of the pumphousing adjacent the pole ring consisting of magnetically permeablematerial and a permanent magnet pole ring driven by the motor andarranged to transmit the motor torque to the first pole ring with littleoperational slip being arranged adjacent and outside this wall,characterized in that means are provided for reducing the flux passingthrough the magnetically permeable separating wall, thereby the slip isincreased.
 2. A centrifugal pump according to claim 1, characterized inthat the magnetic air gap is variable by axial displacement of one polering (12) relative to the other.
 3. A centrifugal pump according toclaim 2, characterized in that the pole ring (12) is arranged so as tobe displaceable relative to the shaft (13) and axially securable.
 4. Acentrifugal pump according to claim 2, characterized in that the polering (12) forms a unit with the shaft (13) and the armature (9) and thatthis unit is arranged so as to be axially displaceable.
 5. A centrifugalpump according to claim 4, characterized in that the shaft is supportedin ball bearings and that the bearing (14) facing the pump housing isspring biased by the spring (16) towards the pole ring (6''), whereasthe second ball bearing (18) is axially adjustable by positivelyconnected elements (18, 21).
 6. A centrifugal pump according to claim 4,characterized in that the motor armature (9) is disposed symmetricallyto the central rotational plane of the stator (6''), when the air gap ofthe pole ring (12) is at a minimum, whereas the motor armature isdisplaced from the optimum central position as the air gap increases. 7.A centrifugal pump according to claim 2, characterized in that the motorhousing (38) is arranged so as to be displaceable relative to the pumphousing (1).
 8. A centrifugal pump according to claim 1, characterizedin that the first pole ring (3) which is integral with the pump runner(2) consists of soft iron and carries a surface layer (4) of a materialof high electrical conductivity.
 9. A centrifugal pump according toclaim 8, characterized in that the layer of the material of highelectrical conductivity is copper and that it also protects the side ofthe rotor (3) facing the separating wall (27).
 10. A centrifugal pumpaccording to claim 1, characterized in that the first pole ring (3)which is integral with the pump runner (2) consists of a magneticmaterial whose coercive force is so small that at least duringacceleration when starting up slip occurs between the first and thesecond pole ring (12).
 11. A centrifugal pump according to claim 1,characterized in that a ring (73, 80) of magnetically non-conductivematerial is associated with bodies (58, 74, 81) of soft iron, whosecircumferential span is smaller than the pitch (75) between adjacentmagnet poles and that the ring (73, 80) can be pivoted through an anglecorresponding to approximately half a pole pitch (75).
 12. A centrifugalpump according to claim 10, characterized in that the magnet poles (54,71) consist of soft iron and that the permanent magnets (50, 70) aremagnetized in the circumferential direction.
 13. A centrifugal pumpaccording to claim 1, having a pole ring with permanent magnets, onepole face of the magnets being arranged facing the air gap and the otherpole face facing a magnetic return path ring, characterized in that themagnetic return path ring (62) has a number of regions (61) of highmagnetic conductivity equal to the number of poles, the said regionsbeing arranged for displacement in azimuth in such a way that in oneterminal position thereof they are disposed between two adjacent polesand there form the magnetic return path between adjacent poles, whereasin their other terminal position they register with the poles and thusdo not present a return path.
 14. A centrifugal pump according to claim1, characterized in that the pole ring carrying permanent magnets (91,95) consists of two rings arranged within each other or adjacent eachother and having the same number of poles and in that the rings arearranged for relative displacement by approximately half a pole pitch,so that in one terminal position the magnetic fluxes are additive,whereas in the other terminal position they partially neutralize eachother.
 15. A motor driven centrifugal pump whose runner forms a unitenclosed in a pump housing, with a soft magnetic pole ring, the wall ofthe pump housing adjacent the pole ring consisting of magneticallypermeable material and a permanent magnet pole ring driven by the motorand arranged to synchronously transmit the motor torque to the firstpole ring, characterized in that means are provided for reducing theflux passing through the magnetically permeable separating wall, wherebythe stalling torque is reduced.
 16. A centrifugal pump according toclaim 15, characterized in that the magnetic air gap is variable byaxial displacement of one pole ring (12) relative to the other.
 17. Acentrifugal pump according to claim 16, characterized in that the polering (12) is arranged so as to be displaceable relative to the shaft(13) and axially securable.
 18. A centrifugal pump according to claim17, characterized in that the pole ring (12) forms a unit with the shaft(13) and the armature (9) and that this unit is arranged so as to beaxially displaceable.
 19. A centrifugal pump according to claim 18,characterized in that the shaft is supported in ball bearings and thatthe bearing (14) facing the pump housing is spring biased by the spring(16) towards the pole ring (6''), whereas the second ball bearing (18)is axially adjustable by positively connected elements (18, 21).
 20. Acentrifugal pump according to claim 18, characterized in that the motorarmature (9) is disposed symmetrically to the central rotational planeof the stator (6''), when the air gap of the pole ring (12) is at aminimum, whereas the motor armature is displaced from the optimumcentral position as the air gap increases.
 21. A centrifugal pumpaccording to claim 16, characterized in that the motor housing (38) isarranged so as to be displaceable relative to the pump housing (1). 22.A centrifugal pump according to claim 15, characterized in that thefirst pole ring (3) which is integral with the pump runner (2) consistsof soft iron and carries a surface layer (4) of a material of highelectrical conductivity.
 23. A centrifugal pump according to claim 22,characterized in that the layer of the material of high electricalconductivity is copper and that it also protects the side of the rotor(3) facing the separating wall (27).
 24. A centrifugal pump according toclaim 15, characterized in that the first pole ring (3) which isintegral with the pump runner (2) consists of a magnetic material whosecoercive force is so small that at least during acceleration whenstarting up slip occurs between the first and the second pole ring (12).25. A centrifugal pump according to claim 15, characterized in that aring (73, 80) of magnetically non-conductive material is associated withbodies (58, 74, 81) of soft iron, whose circumferential span is smallerThan the pitch (75) between adjacent magnet poles and that the ring (73,80) can be pivoted through an angle corresponding to approximately halfa pole pitch (75).
 26. A centrifugal pump according to claim 24,characterized in that the magnet poles (54, 71) consist of soft iron andthat the permanent magnets (50, 70) are magnetized in thecircumferential direction.
 27. A centrifugal pump according to claim 15,having a pole ring with permanent magnets, one pole face of the magnetsbeing arranged facing the air gap and the other pole face facing amagnetic return path ring, characterized in that the magnetic returnpath ring (62) has a number of regions (61) of high magneticconductivity equal to the number of poles, the said regions beingarranged for displacement in azimuth in such a way that in one terminalposition thereof they are disposed between two adjacent poles and thereform the magnetic return path between adjacent poles, whereas in theirother terminal position they register with the poles and thus do notpresent a return path.
 28. A centrifugal pump according to claim 15,characterized in that the pole ring carrying permanent magnets (91, 95)consists of two rings arranged within each other or adjacent each otherand having the same number of poles and in that the rings are arrangedfor relative displacement by approximately half a pole pitch, so that inone terminal position the magnetic fluxes are additive, thereas in theother terminal position they partially neutralize each other.